1. Field of the Invention
This invention relates to vehicle-mounted extensible masts, and in particular to an apparatus and method for carrying wires along a vehicle-mounted extensible mast.
2. Background of the Invention
A number of vehicles currently carry extensible masts, which typically range in extended length from 30 to 58 feet. Such vehicles include electronic news-gathering vans, satellite news vehicles, electronic field-production vehicles, production trailers, military vehicles, mobile automatic teller machine vehicles, medical trailers, and boats, to name a few. The types of apparati mounted on the tops of the vehicle-mounted extensible masts include microwave transmitters and receivers, cameras, lights, RFD frequency antennas, cell phone antennas, scanner antennas, weather instruments, and in the future, satellite antennas. Communication means must be provided back and forth from the vehicle below and the antennas above to provide power, to send antenna and camera aiming instructions, to transmit data, etc.
Existing Designs
The most prevalent communication means currently available is the coiled tube. Referring now to FIG. 1, we observe vehicle 2 with extensible mast 4 mounted to it. Typically the lowest section of mast 4 passes through the vehicle 2 roof and through its cabin, and is securely anchored to the frame of vehicle 2. The mast 4 depicted in FIG. 1 comprises 8 sections: the lowest section anchored to vehicle 2 previously mentioned, and seven additional sections telescopically extensible relative to the lowest section. Thus, in practice, an eight-section antenna whose collapsed length is only approximately seven feet, measures 58 feet in length when fully extended!
Extensible jacks 7 are provided to stabilize vehicle 2 when mast 4 is extended, and an engine kill switch prevents vehicle 2 from moving under its own power if mast 4 is even slightly extended. Vehicle 2 should not be moved if mast 4 is extended due to a number of hazards, including overhead power distribution lines, bridges, and vehicle 2 instability. Control panel 16 is generally mounted to the exterior of vehicle 2, and affords control of the extension and retraction of mast 4, as well as of extensible jacks 7.
FIG. 1 depicts a fairly common equipment configuration mounted to masthead platform 5: camera 6, microwave dish antenna 8, and RF pancake antenna 10. Lights (not shown) would normally be provided to illuminate masthead platform 5 at night, as well as to warn low-flying aircraft.
The communication means between vehicle 2 and the platform 5 equipment depicted in FIG. 1 is coiled tube 12. Coiled tube 12 is stuffed with wires to provide the communication functions required, and is coiled loosely around mast 4 like a lazy boa constrictor. A lower extreme of coiled tube 12 is attached to vehicle 2, and an opposite, upper extreme of coiled tube 12 is attached to masthead platform 5.
In operation, coiled tube 12 resembles a giant "Slinky" spring. In the case of a 58-foot mast, coiled tube 12 would measure approximately 80 feet if pulled straight. The extended length of coiled tube 12 measures up to 58 feet. In its collapsed position, coiled tube 12 retracts into basket 14, which is disposed around mast 4 and attached to the roof of vehicle 2. When viewed from the top, basket 14 is shaped like an annulus having an outside diameter of approximately three feet, and is approximately two feet high.
Currently available coiled tubes 12 are manufactured of resilient synthetic material such as nylon. Consequently coiled tube 12 possesses material memory which urges it to remain in its coiled shape, and to retract if not forcibly extended. Thus, when mast 4 is retracted, both the material memory inherent in coiled tube 12 and gravity help retract coiled tube 12 into basket 14.
The thickness and number of wires which must be stuffed into coiled tube 12 vary depending on the nature of the equipment installed on masthead platform 5. Depending on the thickness of the individual wires, a maximum of approximately 60 wires will fit into a standard 11/4-inch diameter coiled tube 12.
The currently available coiled-tube 12 systems suffer from a number of disadvantages. The first group of problems stem from the length of coiled tube 12 required. As previously mentioned, an 80-foot length of coiled tube 12 is required in the case of a 58-foot mast--this amounts to a coiled tube 12 which is 12 feet longer than the mast itself. The wasted coiled tube 12 material is only the tip of the iceberg: considering that wire may cost $9-$10 per foot, the waste in material using the current coiled tube method is considerable: the excess cost is at least 20 percent in material alone. This wasted coiled tube 12 material problem is exacerbated by the fact that currently available coiled tube 12 is only sold in 100-foot lengths--thus 20 feet must be lopped off and discarded right from the start to yield an 80-foot useable length.
Another series of problems with the coiled-tube 12 method stems from the difficulty of stuffing wire into coiled tube 12. First, coiled tube 12 must be straightened out. This is accomplished by immobilizing one extreme, and pulling on the opposite extreme using a forklift or truck. Approximately 500 pounds of force must be applied (and maintained) to pull (and maintain) coiled tube 12 straight.
Once coiled tube 12 is straight, the individual wires to be installed are combed straight and spiral-taped together into a bundle, a leader line such as fishtape is threaded through coiled tube 12, soapy wire lubrication is slathered onto the wire bundle, and the wire bundle is pulled through coiled tube 12. This wire pulling step is fraught with peril: if even one wire breaks, the wire bundle must be pulled out of coiled tube 12, the offending wire must be removed, the wire bundle must be re-assembled, and the soapy-wire-pulling step repeated. The entire process of stuffing coiled tube 12 typically takes three men approximately four hours to accomplish. The difficulty and riskiness of this process renders it expensive: a stuffed coiled tube 12 costs approximately $2,200.
The difficulty of stuffing coiled tube 12 emphasizes the capacity problem. There are some configurations of wire bundle which a client may desire which are simply too bulky to install in coiled tube 12.
Another problem inherent in the stuffed coiled tube 12 scheme is lack of flexibility. It is not uncommon for a client to request a wire-bundle configuration change in the midst of installation. When this occurs, the entire stuffing process must be repeated.
This lack of flexibility also comes into play during maintenance: if one wire housed within coiled tube 12 were to break, the entire wire bundle must be removed, serviced, and re-installed. Because this maintenance is not typically performed in the field, such wire breakage results in down-time for the vehicle, which can be quite costly, especially during television "sweep" weeks (or during a Superbowl game broadcast, for that matter).
It is fairly common to re-configure a vehicle for a different function. For example, some vehicles are dual-use: they are used both as transmitters and repeaters. If such change in function requires a change of wire bundle, then the masthead platform 5 must be removed in order to swap stuffed coiled tubes 12. Similarly, if the equipment configuration on platform 5 is changed so as to necessitate altering the configuration of the wire bundle, then the mast head platform 5 must be removed in order to swap stuffed coiled tubes 12 (or to reconfigure the wire bundle within the existing coiled tube 12). This process is time consuming, and thus expensive.
As previously mentioned, the length of coiled tube 12 required under the present method is approximately 20 percent longer than the mast it serves. The longer the wire, the greater the voltage drop. Thus, more power (and fuel) is required to transmit signals over 80 feet than 58 feet, leading to energy waste.
When coiled tube 12 is extended by mast 4, not only must force be applied longitudinally to coiled tube 12, but mast 4 is also subjected to considerable twisting moment by coiled tube 12. The combination of coiled-tube longitudinal spring-force, twisting moment, and weight (approximately 85 lbs. for a stuffed 80-foot coiled tube 12) subject mast 4 to substantial wear-and-tear. This wear-and-tear reduces the life of mast components such as seals, keys, and keyways. Needless to say, replacement communication vehicle masts are not cheap.
Another problem with coiled tube 12 is its low service life: currently available coiled tube 12 lasts only a few years. Therefore, it must be replaced (at a cost of around $2,200) every few years.
The coiled-tube 12 method requires an annular basket 14 to store coiled tube 12 when mast 4 is retracted. Basket 14 is generally mounted to the roof of the vehicle, and measures approximately three feet in diameter and two feet in height. Because of its proportions and generous footprint, platform 5 is generally disposed atop basket 14 when retracted, with its equipment still higher. From an overhead clearance point of view, as well as from an aesthetics point of view, this scheme is problematic.
Finally, although coiled tube 12 has been previously analogized to a lazy snake, during extension and retraction it can become a snarling beast, creating tangles by hanging up on itself while extending. Such tangles increase stress on mast 4, and may reduce the life of coiled tube 12 and its wire bundle through over-extension.